Press forming is a method of processing a material blank by pressing a press forming tool against the material blank to be processed to transfer the shape of the press forming tool to the target material blank. This press forming often causes the problem that, after the press formed part is extracted from the press forming tool, a defective shape, so-called springback occurs due to elastic recovery of residual stress in the press formed part, and the press formed part has a shape different from a desired shape.
A degree of springback is largely influenced mainly by strength of a material. In recent years, especially in the automobile industry, a high-strength steel sheet has been increasingly used for automotive parts in view of weight reduction of automotive body. However, the degree of springback is increased as the strength of the material is increased. Thus, to make the shape after springback closer to a designed shape, a skilled person needs to correct the press forming tool several times at a production site and to repeat trial and error. As a result, a production period is prolonged. Thus, reduction of springback is an increasingly important object to reduce the production period and the cost of automobiles.
To reduce springback, residual stress which causes springback needs to be controlled. As a technique of reducing springback by controlling residual stress, a technique disclosed in Patent Literature 1 is known. This technique relates to press forming for a hat-shaped cross section having a punch bottom portion, a side wall portion, and a flange portion. In this technique, a bending radius of curvature is reduced or a formed height is increased at a pre-process of final press forming process at a portion where tensile stress is generated on an edge line from the punch bottom portion to the side wall portion to cause a line length of the hat-shaped cross section at the pre-process to be longer than a line length of a final hat-shaped cross section, and then cause the hat-shaped cross section to be formed into a product shape in the final process, whereby compressive strain is applied in a hat-shaped cross section direction to reduce the tensile stress. Moreover, in this technique, the bending radius of curvature is increased or the formed height is reduced at the pre-process of the final press forming process at a portion where compressive stress is generated in the hat-shaped cross section direction to cause the line length of the hat-shaped cross section at the pre-process to be shorter than the line length of the final hat-shaped cross section, and then cause the hat-shaped cross section to be formed into the product shape in the final process, whereby tensile strain is applied in the hat-shaped cross section direction to reduce the compressive stress.
Additionally, Patent Literature 2 discloses a technique of providing the effect of reducing springback due to angle variation by preliminary bending a bending portion of a punch shoulder portion in the first process, and forming the bending portion with a press forming tool in which the corresponding portion is chamfered in the second process.